I already have answered you support ticket on the subject. For convenience I am pasting my reply below:
The reason for this problem is not directly related to the RadFileExplorer control but to the HTML mark-up of the provided project. What is causing this behavior is the relative positioning of the RadFileExplorer's parent elements.
Please note that by design, the dialogs have an absolute positioned overlay to provide the modality function. According to CSS specification, the offsets of an absolute positioned element are calculated regarding the first relative positioned parent element.
As a solution to this problem I suggest you to remove the relative positioning of the following CSS definitions: .post, #cnt, #ccnt.
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